High Performance Amorphous In0.5Ga0.5O Thin‐Film Transistor Embedded with Nanocrystalline In2O3 Dots for Flexible Display Application

High‐performance, coplanar amorphous In0.5Ga0.5O (a‐IGO) thin film transistor (TFT) on a polyimide (PI) substrate deposited by spray pyrolysis (SP) is reported. The SP a‐IGO film deposited at 370 °C has less than 8% nanocrystalline‐In2O3 dots and a mass density of 6.6 g cm−3. The a‐IGO TFT on PI exhibits linear mobility over 30 cm2 V−1 s−1 and a negligible shift in threshold voltage (ΔVTH = <0.3 V) under positive bias (+20 V) at 60 °C for 1 h. The TFT performance is stable even when bent to a radius of ≈1 mm. The X‐ray photoelectron spectroscopy results of the SP a‐IGO deposited at 370 °C indicate the O‐vacancy (Vo) related defects of 25.79% and hydroxyl (OH) at less than 3%, indicating a good active/gate insulator interface. A seven‐stage ring oscillator made of SP a‐IGO TFTs exhibits a high oscillation frequency (>2.3 MHz) with a low propagation delay time of ≈30 ns per stage at a supply voltage (VDD) of 15 V. The fabricated gate shift register circuit works up to the last stage without any decrement of the input voltage (15 V) with rising and falling times less than 0.8 µs. Therefore, the coplanar a‐IGO TFT by SP deposited at 370 °C is a promising candidate for low‐cost, flexible TFT backplanes of foldable electronics.


Introduction
The study of high-resolution, flexible display backplanes has increased interest since most mobile devices are moving toward flexible electronics.Transparent amorphous oxide semiconductor (TAOS) is an established material to use as an active layer in thin film transistors (TFTs) to overcome the issues manifested in hydrogenated a-Si (a-Si:H) and poly-Si based TFTs.The a-Si:H TFTs suffer from very low mobility (<1 cm 2 V −1 s −1 ) and DOI: 10.1002/aelm.202300169continuous threshold voltage (V TH ) shift during organic light-emitting diode (OLED) operation. [1,2]On the other hand, the poly-Si TFTs utilized for active matrix (AM) OLED has high mobility (>100 cm 2 V −1 s −1 ), but it also has high manufacturing cost and high off-state currents (I OFF ). [1]Note that, the substrate size for low-temperature poly-Si is still Gen 6 glass (1500 mm × 1850 mm), which makes it incompatible with medium and large-area AMOLED displays. [3]In contrast, AOS TFTs have relatively high mobility, large area scalability (Gen 10.5), and ultralow I OFF with low-cost manufacturing.
OLEDs are the current-driving devices and thus a small amount of V TH change in the TFT generates a big issue in the brightness of display pixel uniformity. [2]ince the existing mobile gadgets, such as smartphones, tablets, and laptop computers, are progressing toward foldable devices, it is crucial that the display pixel circuits function flawlessly during and after the flexible substrate is repeatedly folded.6] Therefore, the TAOS is getting significant interest for its application in flexible electronics.The high mobility AOS TFTs suffer from stability issues under bias temperature, [7][8][9] ambient light illumination, [8,9] and mechanical stress, [10,11] which are not suitable for flexible displays.Stabilizer materials like B, La, Hf, Al, W, and Ti [12][13][14][15][16][17] can be added to oxide semiconductors to enhance stability, but it typically reduces mobility and is not suitable for high-resolution, large-area AMOLED displays.
There are several material compositions for TAOS for active semiconductors using indium (In) and tin (Sn) as carrier enhancers and gallium (Ga), and zinc (Zn) as a stabilizer.Popular oxide semiconductors like InGaO, [18][19][20][21][22][23][24] InSnO, [25][26][27][28] InZnO, [29][30][31][32] ZnSnO, [33][34][35][36] InGaZnO, [1,2,[7][8][9][10] InGaSnO, [37][38][39][40] InZnSnO, [41][42][43] and InGaZnSnO [44][45][46][47] are widely studied.The choice of the materials in the periodic table suggests the atoms with spherically symmetric s-orbitals with a comparatively large ionic radius tends to show high mobility. [48]Among them, the a-InGaZnO is extensively studied and used for the display TFT backplane by many companies because of its comparatively high mobility (>10 cm 2 V −1 s −1 ), low-temperature processing (<200 °C), and low off currents.However, its mobility is limited by the hexagonal structure of a-InGaZnO and the use of two stabilizer metal cations (Ga 3+ and Zn 2+ ).Note that, only In 2 O 3 fulfills the requirement of the ionic octahedral edge-sharing electronic configuration (n − 1)d 10 ns 0 (n ≥ 5), which is preferred for higher carrier concentration (N b )/mobility enhancement.In contrast, Zn 2+ primarily stabilizes amorphous (tetrahedral) structures, whereas Ga 3+ can serve both as a carrier suppressor of In 2 O 3 and a stabilizer simultaneously. [49,50]n 2 O 3 itself is not suitable for coplanar TFTs because of its high N b (≥10 19 cm −3 ) and thermal instability attributed to its high O deficiency. [51]Doping it with highly bonded metal cations with high M-O bond dissociation energy (BDE) is an efficient technique to suppress the O deficiency.Therefore, only the insertion of Ga 3+ into In 3+ , which has a higher Ga-O BDE (374 kJ mol −1 ) than In-O BDE (346 kJ mol −1 ), is an appropriate option for regulating the N b of In 2 O 3 by substituting the O-deficiency sites in the In 2 O 3 structure.Additionally, the higher bandgap (4.8 eV) of Ga 2 O 3 than that of In 2 O 3 (3.7 eV), as well as its polyhedral network, which reduces the clustering of In-O polyhedral network and frustrates the In 2 O 3 bixbyite structure.This transforms it into the amorphous phase, making it suitable for use in coplanar TFT applications. [51]he choice of device structure and the deposition process of each layer are also important factors for TFT manufacturing.The back channel etch (BCE) [1,[5][6][7] and coplanar [51][52][53][54][55][56] are the most popular TFT structures used for the display backplanes.The former has fewer mask steps but suffers from the unwanted source/drain (S/D) to bottom gate (BG) overlap capacitance, whereas the coplanar structures provide a negligible overlap capacitance because of their offset structure.Therefore, display manufacturing companies prefer the coplanar structure for the AMOLED display backplanes. [55,56]As for the thin film deposition process several deposition techniques such as sputtering, [1,2,[51][52][53][54] atomic layer deposition (ALD), [23,57,58] spin-coating, [20] and spray pyrolysis (SP) [18,57,58] are widely used for InGaO thin film formation.Reactive sputtering of oxide semiconductors is carried out in high vacuum chambers, which makes them expensive.In contrast, spin coating is not appropriate for large-area substrate manufacturing because of its uniformity issue.The SP thin film deposition can be carried out on a preheated substrate, followed by a cool-down in the air.It is a very simple, affordable method of depositing a very uniform InGaO thin film.However, the substrate surface temperature (T Sub ) should be high enough to evaporate the solvents and to remove H in the film to achieve a high-quality film by SP.
In this work, we studied the SP amorphous In 0.5 Ga 0.5 O (a-IGO) thin film growth at theT Sub of 290, 330, and 370 °C, respectively.The T Sub required to deposit uniform bubble-free a-IGO thin films was found to be ≥330 °C using the 2-methoxy ethanol (2-ME) solvent.The mass density of a-IGO increases from 5.5 to 6.6 g cm −3 , by increasing the T Sub from 290 to 370 °C.It is confirmed that the 370 °C a-IGO thin film has ≈5% to 8% nanocrystalline In 2 O 3 dots inside.The Leiden frost-dominated boiling (LFDB) growth of the SP-IGO thin film exhibits good quality a-IGO film at higher temperatures (330 and 370 °C).Top gate coplanar TFTs were fabricated using the different T Sub (290, 330, and 370 °C), where 370 °C-grown TFTs exhibit superior TFT perfor-mance: V TH , linear field effect mobility (μ FE ), saturation mobility (μ SAT ), subthreshold swing (SS), and on-state (I ON ) to I OFF current ratio (I ON/OFF ) of −0.05 ± 0.54 V, 27.53 ± 2.14 cm 2 V −1 s −1 , 31.36 ± 4.04 cm 2 V −1 s −1 , 156 ± 18 mV dec −1 , and >10 9 A. Whereas, medium performances: V TH , μ FE , μ SAT , SS, and I ON/OFF of 0.08 ± 0.61 V, 15.87 ± 2.74 cm 2 V −1 s −1 , 26.87 ± 2.86 cm 2 V −1 s −1 , 159 ± 19 mV dec −1 , and >10 9 A are found for the TFTs grown at 330 °C.But, the TFT performance using the a-IGO grown at 290 °C does not show a good TFT performance.The TFT performance is related to the film density, Vo, and -OH-related defects.It is found that the negligible ΔV TH under gate voltage stress (V GS-stress ) = ±20 V and 60 °C can be observed for 330 and 370 °C TFT.A negative bias illumination stress under 10 000 nits white light with V GS-stress = −20 V shows a lower ΔV TH shift of −1.1 V at 370 °C grown a-IGO compared to the 330 °C TFT (ΔV TH = −4.9V).It was also observed that the robust TFT characteristics can withstand mechanical bending stress down to an ≈1 mm bending radius.A seven-stage ring oscillator (RO) and a 96-stage gate shift register (GSR) circuits made of the SP a-IGO TFT were demonstrated with a high oscillation frequency (f osc ) of 2.36 MHz and low rising/falling time of <0.8 μs, respectively.

Result and Discussion
The IGO solution with In:Ga = 1:1 was synthesized in an N 2 glove box and a-IGO thin films were deposited using SP in a cleanroom air environment on Glass/200 nm-SiO 2 buffer substrate at the T Sub of 290, 330, and 370 °C, respectively.Figure 1a shows the schematic diagram of the SP system used in this experiment, where the spray nozzle to substrate surface distance and its N 2 gas pressure was kept at 10 cm and 0.2 MPa, respectively.The flow rate of 0.05 mL s −1 with a nozzle speed of 6 mm s −1 was maintained over a 15 cm × 15 cm substrate.The liquid droplets form from the spray nozzle and travel to the top of the preheated substrate by gravitational force through the hot air flux generated from the hot substrate.This transition depends on several factors such as N 2 gas pressure, T Sub , solvent evaporation temperature, and spray nozzle to substrate distance. [59,60][61][62] A good quality SP thin film can be achieved in LFDB, which is required for the liquid solvent evaporation by the preheated substrate heat flux without wetting the substrate. [60,61]or good film deposition using IGO precursor solution by SP, the LFDB can be achieved at the T Sub of 330 °C or higher, and Figure 1b-d shows the scanning electron microscope (SEM) and optical micrographs (see Figure S1a-i, Supporting Information) of the SP a-IGO thin films deposited at T Sub of 290, 330, and 370 °C, respectively.The thin film was grown at 290 °C shows some coffee rings, indicating nucleation boiling is dominant.In this case, the spray droplets are wetting the substrate before evaporation of the solvent.But the thin films deposited at 330 and 370 °C exhibit smooth surface morphology without coffeering, indicating LFDB film growth.This increases the precursor diffusion on the substrate and induces lateral growth of thin film on the substrate.The magnified SEM images of the corresponding films are shown in the insets of Figure 1b-d, indicating high-density thin film formation at 370 °C, which might be due to the enhanced LFDB growth without substrate wetting, Atomic force microscope (AFM) and X-ray reflectometry (XRR) analysis were carried out to understand T Sub effect on the SP a-IGO film morphology.Figure 1e-g exhibits the AFM topographical images of the SP a-IGO films deposited at T Sub of 290, 330, and 370 °C, respectively.The surface roughness of the SP a-IGO thin film is 2.78 nm at 290 °C, which reduces to 0.26 nm at 330 °C.However, because of the presence of nano-crystalline In 2 O 3 dots, it increases to 0.61 nm at 370 °C. Figure 1h-j shows the XRR mass density of the SP a-IGO thin films at T Sub of 290, 330, and 370 °C, respectively.The large increase in mass density from 5.56 to 6.65 g cm −3 can be seen by increasing T Sub from 290 to 330 °C, which further increases to 6.66 g cm −3 at 370 °C.At 330 °C or above the spray droplets are strictly dominated by LFDB, which increases the diffusion length and produces a highly dense, good-quality, amorphous film.
Self-aligned, coplanar SP a-IGO TFTs were fabricated on buffer-SiO 2 /10 μm-PI/CNT-GO/Glass for flexible electronics, and detach the PI substrate from the carrier glass using the carbon nanotube (CNT)/graphene oxide (GO) release layer. [1,63]igure 2a demonstrates the cross-sectional process flow of the SP a-IGO TFTs on the PI substrate (see the Experimental Section for details).Figure 2b,c exhibit the image of the flexi-ble TFTs on the PI substrate and the optical micrograph of a TFT (W/L = 21.45/6.14μm), respectively.Cross-sectional, highresolution transmission electron microscopy(TEM) was used to see the device structure and amorphous SP a-IGO film formation.Figure 2d demonstrates all the metal layers of the channel area with the SiO 2 buffer/a-IGO active/SiO 2 gate insulator (GI)/Molybdenum (Mo) gate for the SP a-IGO TFT deposited at 370 °C. Figure 2e shows the expanded portion of the a-IGO active layer, which shows a quite uniform SP a-IGO/SiO 2 GI interface.The inset of Figure 2e demonstrates the Fast-Fourier Transform (FFT) image of the expanded active IGO layer, verifying the amorphous phase of the SP-IGO.Figure 2f shows the energydispersive X-ray spectrometry analysis of the corresponding expanded TEM image, demonstrating the uniform distribution of the gallium (Ga), indium (In), silicon (Si), and oxygen (O), respectively in the corresponding layers.
To understand the T Sub effect on the SP a-IGO chemical bonding states, X-ray photoelectron spectroscopy (XPS) depth profile was analyzed from the Glass/200 nm SiO 2 /15 nm SP a-IGO stack for the T Sub of 290, 330, and 370 °C, respectively.Indium (In) to gallium (Ga) ratio in the IGO front interface (In:Ga  ages of the M-O, Vo, and ─OH are summarized in Figure 3b,c.The M-O, Vo, and -OH group-related peak areas are 61.33%,35.16%, and 3.52% for the SP a-IGO thin film deposited at 290 °C, and they are 69.01%,27.31%, and 3.69% for the a-IGO deposited at 330 °C.There is a significant increment in M-O bond and a decrement in the Vo area, suggesting more M-O bond formation at higher T Sub 's.Note that, by increasing T Sub to 370 °C, the M-O, Vo, and -OH peak areas are found to be 72.03%,25.79%, and 2.18%, respectively.The M-O concentration is lower in the back interface (65.79%) compared to the front interface (72.03%) at 370 °C.The Vo and -OH-related defects areas are also higher in the back interface.This indicates that the IGO formation has more defects initially and they decrease gradually with increasing thickness.Note that, the film deposition by SP is layerby-layer growth.The first 1-2 spray cycle might have more defects than the later ones because it starts on the SiO 2 buffer layer.OHrelated defects are also higher (3.54%) near the buffer compared to the front interface (2.18%) grown at 370 °C.
Hall measurements were performed on the SP a-IGO thin films at the T Sub of 330, and 370 °C respectively, as shown in Table S1, Supporting Information, to quantify the N b upon increasing growth temperature.The N b , and Hall mobility (μ Hall ) are, 8.34 × 10 17 cm 3 , and 7.8 cm 2 V −1 s −1 , respectively for the 330 °C deposited a-IGO film, whereas film at 370 °C shows higher N b , μ Hall of 1.6 × 10 18 cm 3 , 8.97 cm 2 V −1 s −1 , respectively.It is important to control the N b of the AOSs to the order of 10 17 cm −3 or less to use it for the TFT. [64,65]The N b observed in the SP a-IGO films (>10 17 cm 3 ) is not suitable for TFT application.[68][69][70][71] In this work, we use N 2 O plasma treatment to reduce the N b .Figure 3d shows the TFT performances with and without (w/o) plasma treatment.The TFTs without plasma treatment could not show switching characteristics, conversely, long-time plasma treatment can create unwanted defects in the a-IGO, which can be reduced by N 2 O post-fabrication annealing (PFA).Therefore, N 2 O PFA is also an important process step to improve the TFT performance and stability. [51,54]The red lines in Figure 3d show the TFT performances after N 2 O PFA.
The XPS analysis was performed on the a-IGO films to understand the change in the chemical bonding upon N 2 O plasma treatment, followed by N 2 O PFA. Figures 3e,f [72] There is a peak shift of 1.12 and 0.99 eV without changing the spin-orbit splitting, observed in the Ga 2p and In 3d spectra, respectively after N 2 O plasma treatment.The XPS data for the IGO films were calibrated with C 1s peak at 284.8 eV. [73]The binding energy shifts to the higher energy upon N 2 O plasma treatment, suggesting the creation of V O -related defects in the IGO. [51,54]The electronegativity of the In 3+ is higher than Ga 3+ therefore the electron density is higher around Indium.The increased binding energy suggests the reduction of electron density around Indium, in turn, resulting in the reduction of electron concentration in overall IGO composition.On the other hand, a negative shift in the binding en-ergy for both Ga 2p (−0.38 eV) and In 3d (−0.25 eV) indicates the carrier increment by defects passivation upon N 2 O PFA. [51,54] The room-temperature (RT) photoluminescence (PL) spectroscopy analysis including an energy level diagram is shown in Figure 3g,h for the a-IGO films.The PL spectra were recorded using a 260 nm UV excitation.The Gaussian fit (dotted line) of the corresponding samples can be fitted with four emission peaks centered at ≈319, 389, 440, and 544 nm.The UV region peak centers at (309 and 389 nm) result from the recombination of neutral donors and self-trapping holes. [74]The visible peak at 440 nm (blue) can be denoted by the donor band (created by V O ) to the acceptor band (creates by In/Ga vacancy [V In/Ga ]) transition.The green emission peak at 544 nm was mainly created due to radiative recombination by a photo-generated hole with electrons which is occupied by Vo, acts as a U center denoted to singly ionized oxygen vacancy (V O + ), as shown in the inset of Figure 3g. [75]The PL intensity is higher after N 2 O plasma treatment, suggesting the defect generation such as Vo, V In/Ga vacancy or interstitials, and V O + , however, N 2 O PFA diminishes the defects. [74,75]Therefore, it can be concluded that N 2 O plasma treatment reduces the N b by introducing defects and passivation of O vacancies.In contrast, N 2 O PFA can be used to improve the TFT performance upon reduction of defects, mainly V o + which is mainly responsible for the TFT performance improvement. [75]igure 4a,f,k shows the transfer characteristics (Log I D vs V GS ), gate leakage current (I G ), and μ FE for the TFTs deposited at T Sub of 290, 330, and 370 °C, respectively.The output, hysteresis, and square root drain current (SQRT) I D versus V G = V D characteristics of the TFTs are shown in Figure S4, Supporting Information.The TFT performance parameters are summarized in Table S2, Supporting Information.All the devices exhibit good switching characteristics with low I G (<10 −12 A) and very low I OFF (<10 −13 A), due to the excellent interface between SP a-IGO and SiO 2 GI.No current crowding observed in the output characteristics shown in Figure S4a,d,g, Supporting In-formation, indicates good ohmic contact between the N+ a-IGO and source/drain (S/D) Mo layer.The TFT with a-IGO thin film deposited at 290 °C exhibits poor performance, whereas, the TFT deposited at 330 °C shows much-improved performance with μ FE of 17.51 cm 2 V −1 s −1 and μ SAT of 25.86 cm 2 V −1 s −1 .Increasing the T Sub to 370 °C boosts the TFT performance such as V TH , μ FE , μ SAT , SS, and I ON/OFF to −0.2 V, 26.96 cm 2 V −1 s −1 , 32.92 cm 2 V −1 s −1 , 0.136 V dec −1 , and 1.9 × 10 9 A, respectively, due to more defect passivation and highly dense a-IGO film formation.One order of increment in the N b upon raising the temperature from 330 to 370 °C might be the main reason behind the negative (−Ve) shift of V TH .Because of higher trap states for the TFT deposited at 290 °C, suffers from the clockwise hysteresis shown in Figure S4b, Supporting Information, whereas the 330 and 370 °C deposited TFTs show hysteresis-free performance shown in Figure S4e,h, Supporting Information, respectively.This suggests the passivation of trap states at higher T Sub deposition.SQRT I D versus V G = V D and their linear extrapolation for saturation mobility are shown in Figure S4c,f,i, Supporting Information, for the SP a-IGO TFTs.
To show TFT uniformity, five sets (four edges and middle) of length variation from 4 to 20 μm (20 TFT devices) were chosen from the fabricated 15 cm × 15 cm substrate, and the parameters (μ SAT , μ FE , SS, V TH ) are summarized in Table 1 and the histogram of the devices are shown in Figure 4b-e,g-j,l-o, respectively for the T Sub of 290, 330, and 370 °C.The TFTs deposited at 330 and 370 °C exhibit uniform TFT parameters with small standard deviations in the μ SAT , μ FE , SS, and V TH .The transfer, mobility, and output characteristics data with a constant width of 20 μm with various channel lengths (4, 5, 6, 10, and 20 μm) for the T Sub of 330, and 370 °C are shown in Figure S5, Supporting Information, and the performance parameters are summarized in Table S3, Supporting Information.It is noted that the small standard deviation shown in the histogram of performance parameters (Table 1) is mainly because of the length variation dependency.It is known that the shorter channel lengths tend to show negative V TH because of the comparatively lower total resistance (R TOTAL ) of the TFTs. [76]Table 2 summarizes the performances of the a-IGO TFTs reported in the literature including current results.Using 50% Ga/(In+Ga) shows the highest mobility reported so far.
The TFT deposited at 370 °C shows negative V TH compared to the 330 °C deposited TFTs, which can also be explained by the R TOTAL reduction for the higher temperature a-IGO because of higher N b .By using the transmission line method we can estimate the channel resistance using the length variation transfer characteristics shown in Figure S6, Supporting Information.The R TOTAL 's, for the length 5, 10, and 20 μm TFTs for the 330 and 370 °C are shown in Figure S6a,b, Supporting Information, respectively.It is evident that the R TOTAL is smaller for the 370 °C TFT.The TFT on the PI substrate was fabricated using the SP a-IGO (T Sub = 370 °C).The cross-sectional view, the image of the detached PI substrate from glass through the release layer CNT-GO, and the optical micrograph of a TFT fabricated on PI substrate, respectively can be seen in Figure S7a-d, Supporting Information.Length variation (4 to 20 μm) transfer characteristics and their summarized performance parameters (SS, V TH , μ SAT ) are shown in Figure S7e,f, Supporting Information.The extracted SS, V TH , and μ SAT for the TFTs on a PI substrate also maintain the TFT performance (140 mV dec −1 , 0.2 V, and 32.13 cm 2 V −1 s −1 ) similar to the TFTs on glass.Ann.Temp. [°C] Spray-IGO/SiO  Figure 5a,b,d,e shows the evolution of the transfer characteristics as a function of gate voltage (V GS ) sweep (−10 to +20 V) upon applying a constant V GS for 3600 s at 60 °C.Positive bias temperature stress (PBTS) at V GS-stress = +20 V and negative bias temperature stress (NBTS) at V GS-stress = −20 V were applied to the corresponding TFTs.A negligible V TH shift of ΔV TH = +0.3V and ΔV TH = +0.2V upon applying PBTS on the 330 and 370 °C TFTs can be seen in the transfer characteristics in Figures 5a,d, respectively.Note that, NBTS has no visible degradation for the corresponding TFTs.The negligible ΔV TH under PBTS and NBTS is mainly attributed to the lower trap state density in a-IGO/SiO 2 GI interface.Negative bias illumination stress (NBIS) test at RT under V GS-stress = −20 V and 10 000 nits white light was carried out.The negative ΔV TH for the 330 °C deposited TFTs is much higher (ΔV TH = −4.9V) than that of the 370 °C deposited TFTs (ΔV TH = −1.1 V) exhibited in Figures 5c,f respectively.It is known that the NBIS instability occurs mainly due to the positive charge formation by the ionized V o in the a-IGO/GI interface (V o → V o +/ V o ++ or H + ) under light illumination. [8,54]Higher-temperature deposited films show a higher mass density as well as a lower interface trap density, which would be the reason for this improvement.The reduction of Vo and -OH-related defects and the increment of M-O bonds for higher T Sub might be the reason behind the observed robust, hysteresis-free TFT performances (Figure 3) at T Sub of 330 and 370 °C.The small increment under PBTS and a small shift in NBIS stability at 370 °C-grown SP a-IGO TFTs are in good agreement with the lower defect density and highly dense (6.6 gm cm −3 ) a-IGO film formation.The detached PI substrate from the glass wraps up onto a cylinder with a radius ranging from ≈5 to ≈1 mm to check the TFT performance with various bending stress.Figures 5g,h show the experimental setup and the transfer characteristics after detachment from the carrier glass followed by the application of various mechanical strains, respectively.A very negligible ΔV TH = 0.2 V suggests excellent stability under bending stress.
Figure 6 shows the TFT circuits and their results for the sevenstage RO and 96-stage GSR made of SP a-IGO TFTs deposited at T Sub of 370 °C.circuit.Each stage of the GSR contains six a-IGO transistors and two capacitors as shown in Figure 6e and the timing diagram of a single stage demonstrated in Figure 6f. [51]The six transistors were denoted as T1 (W = 10 μm), T2 (W = 10 μm), T3 (W = 100 μm), T4 (W = 10 μm), T5 (W = 20 μm), and T6 (W = 10 μm), where, T1 and T3 are the input and driving TFTs.The bootstrapping and low-level holding capacitors are denoted as C1 and C2, respectively.All the TFTs in the GSR circuits consist of a constant channel length (L = 6 μm).The measured input-to-output waveforms of the GSR for the TFTs fabricated with T Sub 370 °C are shown in Figure 6g.The GSR circuit fully works up to the 96th stage without any decrement in the input voltage (15 V).Note that, rising and falling times for the GSR made of 370 °C TFTs shows rising time, T r = 0.758 μs and falling time, T f = 0.787 μs at the last stage (96th).

Conclusion
In conclusion, we developed a new deposition technique of a-IGO (1:1) by SP at the T Sub of 370 °C.The effect of T Sub (290, 330, and 370 °C) on the SP-IGO thin film morphology is studied, and found the best T Sub condition for the coplanar IGO TFTs for the application in flexible display backplane.By using the SEM, AFM, XRR, and XPS experiments it is evident that the film surface morphology and the chemical states of the SP a-IGO thin films could be improved at a high T Sub of 370 °C because of the dominant LFDB mechanism during spraying.AFM surface roughness is 2.78 nm at 290 °C and decreases to 0.26 nm at 330 °C, but increases to 0.61 nm at 370 °C.However, the film density continuously increases from 5.56 to 6.65 and 6.66 g cm −3 with increasing T Sub to 370 °C.The fabricated, coplanar TFT performances also exhibit drastic improvement in terms of mobility and SS.The results on PBTS and NBTS for the 330 and 370 °C deposited TFTs are excellent with negligible ΔV TH = <0.3V, due to the small amount of V o and -OH-related defects.The TFT on the PI substrate was also demonstrated with a similar device performance to the glass substrate.The TFT on PI being bent down to a 1 mm radius shows a negligible change in the ΔV TH of <0.2 V.The RO made of SP-IGO provides excellent results of f osc (2.36 MHz) at V DD of 15 V, and the GSR shows lower rising/falling time (<0.8 μs).Therefore, SP a-IGO could replace the current oxide being used for flexible AMOLED displays because of its excellent device performance, stability, and circuit applicability.

Experimental Section
PI Substrate Preparation: First, CNT and GO solutions were prepared separately with deionized water.Then both CNT and GO solutions were mixed at a proportion of CNT:GO = 1:4 and sprayed on a 0.5 T carrier glass substrate to form a very thin release layer.A 10 μm PI layer was spin-coated on the glass/ CNT-GO followed by a multistep (10 °C-5 min per step) hotplate annealing ranging from 80 to 140 °C.A 400 nm SiO 2 buffer layer was deposited on the PI using plasma-enhanced chemical vapor deposition (PECVD).
Precursor Solution Synthesis: Indium (III) chloride {InCl 3 } and gallium (III) nitrate hydrate {Ga(NO 3 ) 3 •xH 2 O} purchased from Sigma-Aldrich was used as indium and gallium precursors, respectively, and 2methoxyethanol (2-ME) used as a solvent for the solution.0.1 m IGO precursor solution at a proportion of In:Ga = 1:1 was synthesized by dissolving the indium and gallium precursors in a 2-ME solvent in the glovebox N 2 environment.After mixing the precursors into the solvent in a 30 mL vial, it was mounted on a magnetic stirrer at 50 °C for 2 h.A 0.45 μm PTFE filter was used to filter out the unwanted particles before use in the SP process.
Spray In 0.5 Ga 0.5 O Thin Film Deposition: As shown in Figure 1a, the spray hotplate was preheated at temperatures 290, 330, and 370 °C, and the substrate was mounted on the hotplate.The hotplate vacuum holes made sure the stability of the substrate on the hotplate without any movement while spraying.Spray nozzle to substrate surface distance, N 2 gas flow pressure was kept constant at 10 cm and 0.2 MPa, respectively to get a continuous spray flow rate of 0.05 mL s −1 where nozzle speed of 6 mm s −1 was maintained to achieve uniform SP a-IGO thin film in a large area (15 cm × 15 cm) substrate.The temperature flux generated from the hotplate usually makes the evaporation of the solvents just before the spray droplets hit the substrate and deposit a uniform SP a-IGO thin film.A total of five spray cycles were used to achieve a thickness of 15 nm.
Device Fabrication: Top-gate, self-align coplanar TFT was fabricated using the SP a-IGO thin films.First, a 400 nm SiO 2 buffer layer was grown by PECVD at 420 °C on the PI substrate.Then, 15 nm SP a-IGO thin film as an active semiconductor layer was deposited on the preheated 0.5 T glass/CNT-GO/10 μm PI/400 nm buffer layer stack substrate with the variation of T sub 290, 330, and 370 °C.N 2 O plasma treatment was performed at 420 °C for the 1800 s at a constant N 2 O flow rate and plasma power of 300 sccm and 20 watts, respectively.A 100 nm SiO 2 GI and 150 nm top gate Mo were deposited by 200 °C-PECVD and RT-sputtering process.To form the gate and GI island photolithography process (photoresist (PR) coating → soft bake → expose → hard bake → strip) up to hard bake was performed and the gate island was formed by the wet etching process using Mo-72 wet etchant.][77] Note that, the N+ IGO sheet resistance of 507.44 ± 40.3 Ω sq.−1 (ten data points on all over the 15 × 15 cm sample) were measured using the four-point probe measurement system, suggesting an excellent N+ a-IGO formation with a N b of 1.71 × 10 20 cm 3 (Table S1, Supporting Information), which was very crucial for good ohmic contact in the coplanar TFT structure.The PR was removed by striping and 300 nm SiO 2 at 300 °C-PECVD was deposited as an interlayer.After the via hole formation up to the N+ a-IGO, a 250 nm Mo layer was deposited by RT-sputtering, and the source/drain was formed by wet etching (Mo-72).Finally, the vacuum annealing at 250 °C for 4 h and N 2 O PFA at 350 °C was performed to improve the stability of the TFTs.[80][81] Thin Film and Device Characterization: The a-IGO thin films were characterized by the GI-XRD (X'Pert Pro), XRR, AFM (Park XE7), and XPS (NEXSA) analysis.All electrical measurements were performed by an Agilent 4156 C semiconductor parameter analyzer in the dark at RT.The V TH was defined as the V GS satisfying I DS = W/L × 10 pA.The μ FE was derived from the transconductance (g m ) with V DS = 0.1 V and the μ SAT was obtained from the saturation region of the TFT transfer curve (I D vs (V GS = V DS )) through the slope of √I D versus V GS = V DS .The subthreshold swing (SS) is taken as (dlog (I DS )/dV GS ) −1 of the range 10 pA ≤ I DS ≤ 100pA, with V DS = 0.1 V.

Figure 1 .
Figure 1.Structural properties of the amorphous In 0.5 Ga 0.5 O (a-IGO) thin films by spray pyrolysis (SP) deposited at the substrate surface temperatures (T Sub 's) of 290, 330, and 370 °C respectively.a) Schematic of the spray equipment used in this experiment.b-d) Scanning electron microscope (SEM) images and the insets show the magnified SEM images of the corresponding films.e-g) Atomic force microscopy topographical images show the surface roughness, and h-j) X-ray reflectometry data show mass density of the a-IGO thin films grown by SP at 290, 330, and 370 °C, respectively.

Figure 2 .
Figure 2. Cross-sectional views of the fabrication process and high-resolution transmission electron microscope (TEM) analysis of a coplanar a-IGO TFT at the T Sub of 370 °C.a) Fabrication process flow of coplanar a-IGO TFT by SP on PI substrate.b) Detached flexible TFT on the PI substrate and c) optical micrograph of the fabricated coplanar a-IGO TFT.d) Cross-sectional TEM image of the coplanar IGO TFT.e) The expanded portion of the active layer and the inset is the Fast Fourier transform image of the corresponding image showing the amorphous phase of the IGO active layer.f) Crosssectional TEM image and energy-dispersive X-ray spectrometry analysis of the corresponding image showing the elements Ga: Gallium, In: Indium, Si: Silicon, and O: Oxygen, respectively.
exhibit the core level Ga 2p and In 3d peaks of the as-deposited, N 2 O plasma treated, and N 2 O PFA thin films, respectively.The spin orbits

Figure 3 .
Figure 3. X-ray photoelectron spectroscopy (XPS) depth profile and photoluminescence (PL) spectroscopy analysis.a) Indium (In) and Gallium (Ga) percentages in the front, bulk, and back interface in the SP a-IGO.Summarized metal-oxide, O-vacancy, and -OH intensities from the deconvolution of the O 1s spectra for a-IGO films deposited at 290, 330, and 370 °C, respectively at b) ≈5 and c) ≈12 nm depth from the interface.d) Effect of N 2 O plasma treatment and N 2 O post-fabrication annealing (PFA) on the SP a-IGO TFTs.e) Ga 2p and f) In 3d core level XPS spectra, and g) PL spectra and the insets are the magnified areas corresponding to singly ionized oxygen vacancy (V o + ) for the as-deposited, N 2 O plasma treated, and N 2 O PFA followed by the plasma treatment SP a-IGO thin films, respectively.h) The schematic diagram of the energy level explains the emission mechanism for the SP a-IGO films.

Figure 4 .
Figure 4. Device performance and uniformity for the coplanar TFTs grown at 290, 330, and 370 °C, respectively for 20 TFTs each.Transfer characteristics, gate leakage currents (gray Lines), and linear field-effect mobility (blue lines) of the coplanar a-IGO TFTs grown at a) 290, f) 330, and k) 370 °C by SP at constant drain voltage (V DS ) of 0.1, 1, 5, and 7.5 V at gate voltage (V GS ) sweep (−10 to +20 V). (b-e), (g-j), and (l-o) show the histogram of 20 TFT device performances including five sets of length variation (4 to 20 μm) parameters summary (μ SAT , μ FE , SS, V TH ) on a 15 cm × 15 cm glass.

Figure 5 .
Figure 5.The stabilities of spray-coated a-IGO TFTs under bias-temperature-light and mechanical bending stress for the a-IGO TFTs deposited at 330 or 370 °C.(a,d) shows the positive bias temperature (PBTS), (b,e) shows the negative bias temperature (NBTS),and (c,f) shows the negative bias illumination (NBIS) stress for the 330 and 370 °C spray deposited a-IGO TFTs, respectively.PBTS and NBTS were performed at 60 °C for 1 h under ±20 V V GS stress.NBIS was performed for 1 h under −20 V V GS stress and 10 000 nits white light.g) Image of the bending (≈1 mm bending radius) experimental setup for the TFTs on the PI substrate and the TFT transfer characteristics of as-fabricated and after detachment of PI from glass, followed by the evolution of the TFT transfer curve under mechanical bending stress with respect the bending radius (5 1 mm) shown in (h).
Figure 6a exhibits the equivalent circuit diagram and optical micrograph of the seven-stage RO circuit and Figure 6b demonstrates the output of the RO circuits.The driving and loading TFTs of an inverter used in each stage of the RO are W/L = 40/4 μm and W/L = 4/4 μm with a beta () ratio of 10, respectively.The output waveform of the RO exhibits a very high f osc (2.36 MHz) and a very low propagation delay time of 30.27 ns per stage at a supply voltage (V DD ) of 15 V. Figure 6c,d shows the fabricated four-clock GSR circuit optical micrograph and the equivalent circuits of the corresponding

Figure 6 .
Figure 6.Ring oscillator (RO) and gate driver (GSR) circuits made of a-IGO TFT grown at 370 °C by SP. a) Equivalent circuit and optical micrograph of the RO made of spray-coated a-IGO TFTs.b) The output characteristics of the seven-stage ROs made of a-IGO TFTs.c-f) The optical micrograph, equivalent circuit, single stage equivalent circuit, and timing diagram of the 6T2C GSR made of a-IGO TFTs grown by spray coating.g) The output of 6T2C GSR is made of a-IGO TFTs.

Table 2 .
Summary of a-IGO TFTs reported in the literatures.Active thickness, TFT structure, Ga%, deposition and annealing temperature, V TH , Mobility, SS are shown.